There are many lightwave applications, such as optical telecommunications and chip interconnects, that involve transmitting optical signals and converting them to electrical signals at high data rates. The systems for performing such transmission and conversion usually require a photodetector compatible with the speed and bandwidth of the optical signal. The typical photodetectors are PIN (p-type/intrinsic semiconductor material/n-type) semiconductor detectors.
To date, it has been a challenge to make a Si-based semiconductor PIN photodetector with a bandwidth of 10 GHz or greater. Conventional discrete PIN Si detectors operate at speeds of 2 GHz or less because of their relatively low absorption coefficient and low carrier collection efficiency. The best Si detector known today is the interdigitated lateral trench device (LTD), which operates at speeds of up to 6.5 GHz due to improved absorption by the trench structure.
It is well known that excess optical power density in PIN photodetectors causes detector speed degradation. This is especially true for waveguide-based PIN photodetectors because light is coupled in to a small region about the size of the waveguide. As a result, the detector may not be able to operate at high photocurrent where high-speed operation may require a high current. Where such a system employs evanescent coupling to the intrinsic region of the PIN detector, the intrinsic region can be expanded to dilute the optical power, which in turn prevents the creation of excess carriers. However, the light distribution in the expanded waveguide region (i.e., the waveguide plus the intrinsic region) is not uniform so that the detector electrodes need to be made relative large to ensure adequate detection of the photon-generated carriers. Unfortunately, the relatively large electrode area results in a relatively high detector capacitance, which reduces detector speed. Further, the non-uniform distribution of light in the expanded waveguide region can result in high optical fields, which generate local excess photon-generated carriers. This reduces the detector speed when the excess carriers have to diffuse out of the local excess carrier area to be collected by the electrodes.
Further, photon-generated carriers formed in the intrinsic region of a PIN detector may collected either via electrodes in the top or bottom of the detector as discussed above, or by metal-semiconductor-metal (MSM) interdigitated electrodes on the surface of the intrinsic region. In the first design, the carrier collection distance is set by the required minimum detector thickness for efficient light absorption due to evanescent coupling with the waveguide. This thickness, however, limits the detector speed. In the latter design, photon-generated carriers also have to travel to the interdigitated electrodes across the detector, so that the detector thickness also limits the detector speed.